Proposal for the Detection of Magnetic Monopoles in Spin Ice via Nanoscale Magnetometry*

Since the origin of the “monopole” concept in spin ice, there has been an intensive search for a direct experimental signature of these emergent quasiparticles. Despite various attempts, to date no such signature has been found. We present a proposal for applying nanoscale magnetometry to the search for magnetic monopoles in the spin ices holmium and dysprosium titanate. Employing Monte Carlo simulations of the dipolar spin ice model, we find that when cooled to below 1.5K these materials exhibit a sufficiently low monopole density to enable the direct observation of magnetic fields from individual monopoles. We demonstrate that noise spectroscopy can capture the intrinsic fluctuations associated with monopole dynamics, allowing one to isolate the qualitative effects associated with both the Coulomb interaction between monopoles and the topological constraints implied by Dirac strings. We describe three different nanoscale magnetometry platforms (muon spin rotation, nitrogen vacancy defects, and nanoSQUID arrays) that can be used to detect monopoles in these experiments, and analyze the advantages of each.